CBS report revives claim of an Obama cover-up of the assault that killed the US ambassador to Libya and three others

A former Guantánamo Bay inmate masterminded the fatal attack on the US consulate in Benghazi, it was claimed on Monday, as critics of the President Barack Obama revived accusations that the White House was hiding the truth about the incident.

An investigation by CBS News claimed that Sufyan Ben Qumu, a Guantánamo detainee who was released in 2007 by the Bush administration, was one of the lead planners of the Benghazi assault in September 2012.

Qumu was sent back to his native Libya despite a Department of Defence assessment that he was "medium to high risk, as he is likely to pose a threat to the US, its interests and allies."

He was released by Col Muammar Gaddafi's regime and lived openly in eastern Libya, near the scene of the consulate attack.

Intelligence officials have disputed previous reports that Qumu was behind the assault, which killed Chris Stevens, the US ambassador, and three other Americans on September 11, 2012.

Lindsey Graham, a leading Republican senator, is threatening to prevent Mr Obama from making key government appointments unless survivors of the attack appear before Congress.

"I'm tired of hearing from people on TV and reading about stuff in books.

We need to get to the bottom of this," Mr Graham told Fox News.

Several CIA officers who survived the attack have already briefed both House and Senate intelligence committees but Mr Graham is demanding that the State Department produce a list of the staff it had on the ground.

In a letter to John Kerry, the Secretary of State, Mr Graham threatened to "use all available procedural tools to ensure appropriate oversight committees are able to speak with the survivors".

Under Senate procedures a single senator can slow down the appointment of key government posts - like ambassadors and cabinet secretaries - and in some cases block them altogether.

Among the nominees will soon be before the Senate is Janet Yellen, Mr Obama's nominee to head the Federal Reserve and take up one of the most important posts in global economics.

A British security contractor, using the pseudonym Morgan Jones "for his own safety", told CBS he warned the US a year before the attack that the militia men hired to protect the consulate were not up to the task.

"I was saying 'these guys are no good. You need to get them out of here'," he said.